Comparing the behaviour of spherical beads and natural grains in bedload mixtures

Abstract

It is common to use idealised materials to study the dynamics of granular transport in fluid flows, but the impact of this choice upon sediment behaviour has not been extensively explored. To tackle this research gap, two experiments were undertaken to explore the influence of a finer grain input to a channelized coarser granular flow driven by a shallow fluid flow. The first set of runs was undertaken using spherical glass beads, and the second set with natural fluvial sediment. The transport system approximates a narrow slice through the bedload at the bottom of a river. In the runs with natural grains, the infiltration of fine sediment into the bed was similar to the spherical glass beads, but with reduced infiltration capacity. We ascribe this behaviour to irregular and variable pore shapes and sizes in the natural material. The behaviour of the bedload in the natural material runs matched that of the bead runs only when the feed contained a high content of fines. When the feed contained a low content of fines the transport of natural grains was more complex, including the emergence of migrating collections of grains. However, the overall changes in bed and water slope due to the finer grain input were comparable in both sets of experiments. We conclude that artificial, idealised materials qualitatively represent sedimentary phenomena, but that quantitative differences in the outcomes must be expected. © 2020 John Wiley & Sons, Ltd.

Bridging the gap between particle-scale forces and continuum modelling of size segregation: application to bedload transport

“…2015; Dudill, Frey & Church 2017; Dudill et al. 2018, 2020). While investigating segregation at the granular scale (usually with discrete methods) is invaluable (Hill & Tan 2014; Ferdowsi et al.…”

“…The physics of granular media has been advocated to address segregation at the granular scale and understand geomorphological evolution (Frey & Church 2009. Size segregation largely originates from local interparticle interactions but has huge consequences for the particle size repartition both in the downward and streamwise directions over a much larger scale, potentially affecting sediment mobility and the entire channel geomorphological equilibrium (Gilbert & Murphy 1914;Ferguson et al 2015;Dudill, Frey & Church 2017;Dudill et al 2018Dudill et al , 2020. While investigating segregation at the granular scale (usually with discrete methods) is invaluable (Hill & Tan 2014;Ferdowsi et al 2017;Chassagne et al 2020b), it is also necessary to consider continuum modelling to improve our theoretical understanding and to provide predictions at larger scales.…”

Discrete and continuum modelling of grain size segregation during bedload transport

“…Sediment grain interactions can produce vertical, longitudinal or lateral sorting, which lead to very complex and varied morphologies of bed surface and subsurface (such as armouring, bedload sheet, etc.) (Dietrich et al 1989;Recking et al 2009;Frey & Church 2011;Bacchi et al 2014), and can drastically modify the fluvial geomorphological equilibrium (Dudill, Frey & Church 2017;Dudill et al 2018Dudill et al , 2020.…”

The impact of intermittency on bed load sediment transport

“…The mean water depth was H = 0.1 m and the mean hydraulic radius was R = W H/(W + 2H) = 0.005 m. This corresponds to a Reynolds number Re = uR/ν ≈ 4800 and a Froude number Fr = u/ √ gH ≈ 1. Grains were fed into the flume at a fixed rate by a 'tinker feeder' grain hopper 64,65 , making the sediment flux a fixed input parameter in our experiments. Once inside the flume, the grains accumulated, forming an aggrading bed until steady-state was reached, at which point the bed stopped aggrading and grains exited the flume, where they were collected by a sediment trap.…”